1. Field of the Invention
This invention relates to the transmission of digital information, and more particularly to the use of correlative level coding techniques in combination with digital processing and analog modulation to generate a single-sideband signal.
2. Background Description
The three basic techniques which may be used in analog modulation processing for the generation of single-sideband signals are well-known. These three basic techniques are described in an article by Donald K. Weaver, Jr., "A third method of generation and detection of single-sideband signals," proceeding of the IRE, Vol. 44, No. 12, December 1956, pages 1703-1705. Such techniques have not been previously applied to digital signals. Memoryless techniques are out of the question for single-sideband signals since they contain dc and most of the energy is concentrated near the low frequency end of the spectrum. To remedy this disadvantage would require elimination of the dc and low frequency at the transmitter plus the use of quantized feedback at the receiver in order to re-introduce the dc and low frequency components. This would require techniques which are too cumbersome and complex for a multi-level waveform.
The modulation techniques which have been employed in digital transmission systems in the prior art are generally referred to as amplitude-shift keying (ASK), phase-shift keying (PSK) often referred to as phase modulation or frequency-shift keying (FSK). None of these digital modulation techniques have been amenable to single-sideband transmission. Because of this, a part of the improvement in bandwidth compression which may be obtained through phase-modulation or correlative level coding techniques is lost since the signal generated for transmission in the prior art systems is in effect double-sideband.